The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Internal combustion engines, especially automotive internal combustion engines, generally fall into one of two categories, spark ignition engines and compression ignition engines. Traditional spark ignition engines, such as gasoline engines, typically function by introducing a fuel/air mixture into the combustion cylinders, which is then compressed in the compression stroke and ignited by a spark plug. Traditional compression ignition engines, such as diesel engines, typically function by introducing or injecting pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke, which ignites upon injection. Combustion for both traditional gasoline engines and diesel engines involves premixed or diffusion flames that are controlled by fluid mechanics. Each type of engine has advantages and disadvantages. In general, gasoline engines produce fewer emissions but are less efficient, while, in general, diesel engines are more efficient but produce more emissions.
More recently, other types of combustion methodologies have been introduced for internal combustion engines. One of these combustion concepts is known in the art as the homogeneous charge compression ignition (HCCI). The HCCI combustion mode comprises a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry, rather than by fluid mechanics. In a typical engine operating in HCCI combustion mode, the cylinder charge is nearly homogeneous in composition, temperature, and residual level at intake valve closing time. Because auto-ignition is a distributed kinetically-controlled combustion process, the engine operates at a very dilute fuel/air mixture (i.e., lean of a fuel/air stoichiometric point) and has a relatively low peak combustion temperature, thus forming extremely low NOx emissions. The fuel/air mixture for auto-ignition is relatively homogeneous, as compared to the stratified fuel/air combustion mixtures used in diesel engines, and, therefore, the rich zones that form smoke and particulate emissions in diesel engines are substantially eliminated. Because of this very dilute fuel/air mixture, an engine operating in the auto-ignition combustion mode can operate unthrottled to achieve diesel-like fuel economy.
At medium engine speed and load, a combination of valve profile and timing (e.g., exhaust recompression and exhaust re-breathing) and fueling strategy has been found to be effective in providing adequate heating to the cylinder charge so that auto-ignition during the compression stroke leads to stable combustion with low noise. One of the main issues in effectively operating an engine in the auto-ignition combustion mode has been to control the combustion process properly so that robust and stable combustion resulting in low emissions, optimal heat release rate, and low noise can be achieved over a range of operating conditions. The benefits of auto-ignition combustion have been known for many years. The primary barrier to product implementation, however, has been the inability to control the auto-ignition combustion process.
In a spark-ignition, direct-injection (SIDI) engine capable of operating in an auto-ignition combustion mode (SIDI/HCCI engine), engine air flow is controlled by either adjusting an intake throttle position or adjusting opening and closing times and/or profile of intake valves, using a variable valve actuation (VVA) system. An SIDI/HCCI engine having VVA, e.g., one comprising multiple-step cam lobes which provide two or more valve lift profiles, typically operates in the auto-ignited combustion mode at part-load and lower engine speed conditions and in a conventional spark-ignited combustion mode at high load and high speed conditions. These two combustion modes, however, require quite different engine operation to maintain robust combustion. For instance, in the auto-ignited combustion mode, the engine operates at lean air-fuel ratios with the throttle fully open to minimize engine pumping losses. In contrast, in the spark-ignition combustion mode, the throttle is controlled to restrict intake airflow and the engine is operated at a stoichiometric air-fuel ratio.
There is a need to have a smooth transition between these two combustion modes during ongoing engine operation, in order to maintain a continuous engine output torque and prevent any engine misfires or partial-burns during transitions.
Therefore, there is a need to control the airflow to the engine precisely, to prevent engine operation that is either too lean or too rich during transitions. It is therefore very important that there be coordination between throttle position and operation of the VVA system.
It is further desirable to control any transition between the combustion modes to achieve robust and stable combustion, low emissions, optimal heat release rate, and low noise during the transition.
The invention described hereinafter comprises a method and a control scheme to determine a preferred combustion mode for operating the engine, and controlling the engine thereto.